The scaling of semiconductor devices, such as dynamic random access memory (DRAM), logic devices, and the like, may be limited by gate leakage (Jg). For example, as thickness of a gate dielectric layer is scaled, current may leak between the channel and the gate of a transistor device causing device failure. The gate leakage may be reduced by incorporating nitrogen into the gate dielectric layer. For example, a gate dielectric layer at the 32 nm node may comprise silicon oxynitride (SiON), where the presence of nitrogen reduces gate leakage in the device.
Typically, nitrogen is incorporated into the gate dielectric layer by a plasma nitridation process that provides for gate leakage reduction at the expense of other desired properties, for example, flat band voltage (Vfb), threshold voltage (Vt), and mobility. For example, increased nitrogen content in the gate dielectric layer may undesirably increase Vt and excessively decrease mobility. Further, oxygen may diffuse from the gate dielectric layer under typical processing conditions, thus further reducing device performance, for example by degrading the dielectric properties of the gate dielectric layer.
Furthermore, nitridizing a dielectric layer on a semiconductor wafer for use in a semiconductor structure involves adding nitrogen to a planar semiconductor structure using plasma nitridation or thermal nitridation. However, the use of 3-dimensional (“3D”) semiconductor structures, such as a FinFET device or the like, requires a nitridized layer to wrap around the 3D semiconductor structure with the amount of nitrogen incorporated on the top surface of the 3D semiconductor structure substantially equal to the amount of nitrogen incorporated down the sidewalls of the 3D semiconductor structure, referred to herein as conformality. Conformality is calculated as the percentage of nitrogen drop with depth down the sidewall of the 3D semiconductor structure.
One method of forming a nitridized layer is via thermal nitridation using ammonia (NH3). While thermal nitridation using ammonia (NH3) provides suitable conformality, the process fails to provide the desired nitrogen profile at the top surface of the dielectric layer. Another method of forming a nitridized layer is using inductively coupled plasma nitridation with ions formed from nitrogen gas (N2). While this approach provides the desired nitrogen profile in the dielectric film, the resulting conformality is inadequate. While another method, remote plasma nitridation, can provide suitable conformality, the process requires temperatures in excess of about 600 degrees Celsius to about 1000 degrees Celsius, resulting in excessive and undesirable thickening of oxide layers in the gate stack.
Accordingly, the inventors have provided methods of forming nitrogen containing layers having improved conformality.